1. Technical Field of the Invention
The present invention relates generally to a servo control system for a head which may be employed for recording, reproducing, or erasing information on or from a magnetic or magneto-optical disk. More particularly, the invention relates to a digital servo control system for a magnetic or optical head wherein error signals generated are processed so as to maintain the head to a desired position as commanded by the servo system.
2. Background Art
An optical disk as well as a magnetic disk is provided with a high density and large capacity storage medium. A track-to-track pitch of the disk is very small (1-2 .mu.m), and the rotating disk is subject to change due to eccentricity or slight surface curvatures thereof. It is therefore, necessary to position an optical spot on a target track on the disk with high accuracy.
Generally, an optical disk device has an optical prism which directs light radiated from a semiconductor laser source to an objective lens integrally formed in an actuator to focus a micro-optical spot on the disk. The positioning control of the optical spot is accomplished by monitoring a focal point of the spot and its position in a radial direction of the disk with an optical detector to servo-control the objective lens actuator. The objective lens actuator is, however, usually movable within a small range of .+-.0.1 mm to .+-.1 mm. and is difficult to displace the objective lens toward a desired recording area on the optical disk which has a radius ranging over several tens of millimeter. For overcoming this problem, an arrangement is generally used in a conventional system wherein the objective lens actuator is mounted on a carriage and a linear motor is also attached to the carriage to move the carriage together with the actuator in a radial direction of the disk. This radial movement of the carriage is accomplished with a double-action servo control system. This system is such that the linear motor is activated to move the objective lens greatly, while the objective lens actuator is activated to move same slightly.
Hereinafter, a conventional double-action servo control system will be described with reference to FIGS. 9 to 11.
In past optical head control systems, the servo control using analog signals were usually employed. In recent yeas, with speedup of digital operation, low-cost systems become available and the digital servo control is commonly employed to make digital conversions in part of a servo control loop. The digital servo control is expected to have a further wide range of application in view of the advantages in that it may not be adversely affected with circuit noise or offset in the servo control loop, is free to set or modify various controlled parameters, and has a compact construction with high reliability.
In the optical head control, focusing, tracking, and objective lens-trace servo controls utilize part of control signals of each other, however, have basically different servo control loops respectively. Hereinafter, of the servo control loops, a tracking control circuit for the objective lens actuator will be discussed.
FIG. 9 shows a block diagram of the tracking control system. This system includes generally an optical head driving unit 3, an optical unit 6, a track position detecting circuit 23, an error detecting circuit 24, an A/D converter 25, an arithmetic operation circuit 26, a D/A converter 27, and a driving amplifier 28. The optical head driving unit 3 includes an objective lens actuator and a linear motor. The objective lens actuator disposes therein an objective lens 4 which gathers light onto a recording film of an optical disk 1, and is designed to displace the objective lens 4 slightly. The linear motor is operable to move the objective lens 4 together with the objective lens actuator in a direction over a range from an inner periphery and an outer periphery of the optical disk 1. The optical unit 6 includes therein a semiconductor laser, some prisms, and a combination of an optical element and a sensor which monitors tracking errors and focusing errors. The track position detecting circuit 23 converts a light signal provided by the optical element of the optical unit 6 into a position detecting signal indicative of a position of an optical spot relative to a track on the disk 1. The error detecting circuit 24 is responsive to the position detecting signal from the track position detecting circuit 23 to provide a tracking error signal for the tracking servo control which is, in turn, converted by the A/D converter 25 having a sample holder into a digital signal of 5 to 16 bits. This digital signal is processed in the arithmetic operation circuit 26 which is generally provided with a digital signal processor (DSP) and a microprocessor. The signal processed in the circuit 26 is then converted by the D/A converter into an analog signal again, and also converted in the driving amplifier 28 into an actuator driving signal for position control of the objective lens in the radial direction of the disk 1.
FIG. 10 shows a flowchart of a program or sequence of logical steps performed by the arithmetic operation circuit 26.
The arithmetic operation circuit 26 performs offset control operation (in step 110), gain control operation (in step 120), and filtering operation (in step 130). The offset control operation is such that an offset of low-frequency (DC) components of the A-D converted digital error signals is modified, and is performed prior to correction of the position detecting signals from the track position detecting circuit 23 and mathematical operation by a digital filter. The gain control operation is to set a loop gain required for the tracking servo control. The filtering operation serves to compensate mechanical properties of the objective lens actuator, and is operable to compensate a phase delay or a phase advance for servo system stability. With these operations, the tracking error signal is converted into a feedback control signal for the objective lens actuator as a control signal for the objective lens 4.
While the above discussion is referred to the tracking servo control circuit, the focusing servo control and the objective lens-trance servo control also utilize similar servo control circuits independently of each other. However, provision of an arithmetic operation circuit for each control circuit requires microprocessors of the same number as the control circuits, resulting in increased system costs as well as a bulky circuit.
FIG. 11 shows an example of arithmetic operations performed in a time-division fashion in the focusing servo control, the tracking servo control, and the objective lens-trace servo control by the linear motor. These three operations are performed at a sampling cycle T. Within the sampling cycle T, t.sub.1n, t.sub.2n, and t.sub.3n represent operating time periods of the focusing (Fo) servo control, the tracking (Tr) servo control, and the linear motor (LM) traced servo control respectively. In addition, within the time period t.sub.2n. t.sub.2a, t.sub.2b, and t.sub.2c represent operating time periods of a data sampling operation such as A/D conversion, an arithmetic operation, and an output operation such as D/A conversion. With this time-division control, the three servo control operations are simultaneously performed in a single arithmetic operation circuit, and an optical head digital servo control system of a simple arrangement may be provided.
In the digital servo control, a sampling frequency depends upon control ability and a type of controlled system. In a conventional data storage optical disk device, an optical disk is turned at 1,800-3,600 rpm. In order to have the objective lens follow displacement of the optical disk due to surface curvatures or eccentricity of the optical disk, a sampling frequency of 20-100 is generally necessary. When the sampling frequency is below these values, it becomes difficult for the objective lens actuator to follow the displacement of the optical disk caused by the surface curvatures or the eccentricity thereof with the result that data cannot be recorded on or reproduced from the optical disk in a correct manner and, in addition, the system may not operate normally. Therefore, it is necessary to set the sampling cycle T to a short cycle of 10 .mu.s. For performing the three operations at the same time within the sampling cycle T of 10 .mu.s, each operation must be done within a period of time of several .mu.s. In recent years, a DSP appears on the market as a high-speed microprocessor. However, it is not always easy to perform the data sampling and arithmetic operations within that period.
As previously mentioned, the digital servo control is generally available for producing high-performance, high-reliability, and simplicity of the optical disk. This is based on the primary factor that it is easy to optimize and modify controlled parameters at any time which are used in the offset control, the gain control, and the filtering operations. The controlled parameters are determined based on recording and reproducing signals, condition signals from such as a temperature sensor arranged around the optical head as well as analysis of the focusing position detecting signal and track position detecting signal derived from the optical element in the optical unit 6. It is therefore, necessary for the arithmetic operation circuit to analysis the recording and reproducing signals and the condition signals and to calculate an optimum controlled parameter.
Further, in addition to the above servo control, the arithmetic operation circuit needs to control circuits of peripheral devices, handle recording and reproducing signals, and monitor a malfunction of the system. In addition, the servo control stability and accuracy require adaptive control to modify controlled parameters of the servo control system sequentially in response to disturbances such as variation in environmental conditions for optimization of the control. This must be done in the arithmetic operation circuit.
However, when the arithmetic operation circuit must devote its almost all operating time only to the servo control of the objective lens and the linear motor, it is difficult to incorporate these control operations to the arithmetic operation circuit. This results in the advantage of the digital servo control being reduced.